It has been pointed out that the exhaust gas discharged from internal combustion engines in vehicles such as buses or trucks, construction machines and the like contains a large number of fine particulates, and causes a harmful effect on environment and the human body. Therefore, it has been required to remove and purify the particulates. In order to fulfill such requirement, a filter for purifying the exhaust gas, for example, a filter with a honeycomb structure comprising porous ceramics has been developed.
FIG. 6 is an example of the conventional ceramic filter with a honeycomb structure. The conventional filter is constituted with a cylindrical-shaped honeycomb structural body 30 formed by arranging a plurality of cells 31 as an exhaust gas path side by side in the longitudinal direction through cell walls 33.
As shown in FIG. 6(b), the cells 31 are plugged at either one end portions of inlet side or outlet side for the exhausted gas with plugging materials 32, in which the exhaust gas flown into certain cells 31 passes through the cell walls 33 separating these cells 31 and flows out from another cells 31.
When such a ceramic structural body 30 is placed in an exhaust path of an internal-combustion engine, particulates in the exhaust gas discharged from the internal-combustion engine are caught by the cell walls 33 when passing through the honeycomb structural body 30, and as a result, the purification of the exhaust gas is conducted.
As a filter material for such a honeycomb structural body may have hitherto been used oxides such as cordierite and the like, carbides and the like. Among them, silicon carbide has advantages that it is excellent in the thermal conductivity, heat resistance, mechanical properties, chemical resistance and the like.
For instance, JP-A-S60-264365 discloses a porous silicon carbide sintered body having a three-dimensional net-like structure mainly composed of plate crystals with an average aspect ratio of 2-50.
JP-A-H04-187578 discloses a method of producing a porous silicon carbide sintered body by mixing α-type silicon carbide powder having a mean particle size of 0.3-50 μm and β-type silicon carbide powder having a mean particle size of 0.1-1.0 μm to form raw powder and firing the raw powder.
JP-A-H05-139861 discloses a method of producing a β-type porous silicon carbide sintered body by mixing silicon carbide powder having a mean particle size of 0.5-100 μm and β-type polycrystalline silicon carbide powder having a mean particle size of 0.1-5 μm to prepare raw powder and firing the raw powder.
JP-A-H06-1822282 discloses a method of producing a catalyst carrier by shaping and firing silicon carbide powder having a specific surface area of 0.1-5 m2/gr and impurity components of 1.0-5%.
JP-A-H09-202671 discloses a method of producing a silicon carbide honeycomb filter by mixing α-type silicon carbide powder having a mean particle size of 0.3-50 μm, β-type silicon carbide powder having a mean particle size of 0.1-1.0 μm and the like to form raw powder and firing it.
JP-A-2000-16872 discloses a method of producing a porous silicon carbide sintered body by mixing α-type silicon carbide powder having a mean particle size of 5-100 μm and α-type or β-type silicon carbide powder having a mean particle size of 0.1-1.0 μm to form a mixture and firing it.
JP-A-2001-97776 discloses a porous silicon carbide sintered body and the like wherein silicon carbide crystal particles constituting a porous structure are connected to each other through neck portions and the neck portion smoothly curves.
Generally, a filter for purifying the exhaust gas is subjected to a regeneration treatment in order to burn and remove particulates after catching a certain amount of particulates. However, when a filter made of silicon carbide is subjected to the regeneration treatment, large cracks may be generated in the filter itself due to thermal stress generated in the regeneration treatment. The filter having the cracks has a problem that the exhaust gas leaks out from the cracks and the catching of the particulates becomes incomplete after a long-term use of the filter. Such cracks can occur across silicon carbide particles and cause breakage of the filter.
Further, JP-A-2002-201082 discloses a porous honeycomb structural body for a filter including fire-resistant particles such as silicon carbide particles and the like, and metallic silicon. In such a honeycomb structural body, a catalyst is carried which acts to lower activate energy for the combustion of particulates or conversion harmful gas components such as CO, HC, NOx and the like. Further, as the degree of dispersion to the honeycomb structural body becomes higher, the reaction site to the particulates and the harmful gas components increases and the activity also increases. At a high temperature, however, the specific surface area of the catalyst carrier used for increasing the dispersion degree of the catalyst such as alumina and the like decreases and the sintering of the catalyst itself is caused. Consequently, it is known that the dispersion degree gets worse. Besides, in such a honeycomb structural body, the thermal conductivity is low as compared with the honeycomb structural body made only of silicon carbide, so that when the same amount of particulates is burnt at the regeneration treatment, the activity of the catalyst carried may decrease because heat from burning portions of particulates on surfaces of cells and the like is hard to disperse and the temperature of the burning portions becomes extremely high. Therefore, in the honeycomb structural body including heat-resistant particles and metallic silicon, visible-size cracks may be generated at the regeneration treatment.
It is an object of the invention to solve the above problems inherent to the conventional techniques, and to provide a ceramic sintered body having a long-term stability which can prevent cracks from occurring due to the breakage of ceramic particles when thermal stress is applied and catalyst carried from deteriorating when thermal stress is repeatedly applied, and a ceramic filter produced by using the ceramic sintered body.